1. Field of the Invention
This invention relates generally to circuits for energizing gaseous discharge lamps at high frequency, and more specifically, to such circuits employing SCR-type thyristors.
2. Description of the Prior Art
Conventional ballasting circuits for gaseous discharge lamps are well-known for providing proper voltage for starting and limiting the current during operation. Such ballast circuits are usually large and relatively expensive and generally they are not efficient at low cost. Simple inductor ballasts are available; however, they provide poor regulation for line voltage variations.
Although regulating solid state ballasts have been developed, heretofore no commercial ballast circuits have been developed which are entirely suitable for operating gaseous discharge lamp at high frequency.
Theoretically a lamp may be operated either with ac or with a combination of applied dc and ac. Operation with pure ac has conventionally caused audible, oftentimes annoying noises. Combined ac and dc energization gives lower noise than ac alone, but the application of dc inhibits lamp efficiency and shortens life. The application of low audio frequency ac causes noisy ballast conditions. The application of medium frequency ac causes noisy and unstable lamp conditions. In this regard, the high pitch whine of lamps operated under such conditions can be extremely unpleasant.
It is not surprising, therefore, that there have been many proposals for operating gaseous discharge lamps, such as fluorescent lamps, at high frequency. Such previously proposed high frequency energizing circuits generally have utilized especially constructed transformers and coils and have utilized solid state switching devices. The solid state ballast circuits which have been developed, however, have not been for high frequency, such as well above 20 KHz, operation.
One drawback contributing towards the lack of development of a solid state ballast for operating at high frequency is that the characteristics of conventional transistors are less predictable at high frequency operation than for lower frequency operation. It is believed that thyristors, such as silicon controlled rectifiers or SCR's, have only rarely been utilized because of the finite minimum time during which the SCR drive must be reverse biased to assure latching of the SCR in the non-conductive state. Unless the gate and anode energization is removed for at least this minimum time, the SCR will conduct even without gate energization when a positive voltage is placed on its anode. As the frequency of operation increases and this minimum time becomes a greater and greater portion of the operating cycle, providing this minimum time for naturally commutating the SCR becomes increasingly difficult. Consequently the SCR's have historically been considered relatively low frequency devices.
Many of the advantages which are achieved by operating gaseous discharge lamp lighting systems at high frequency are well recognized. Lumen efficiency is generally acknowledged to increase for both fluorescent and high intensity discharge (HID) lamp systems at higher frequencies of operation. It has even been suggested to be desirable to operate fluorescent-type systems at 50 kHz. The rationale for operating fluorescent-type systems at 50 kHz is based on economic considerations, i.e., based on lamp-efficiency increase and the reduction in size and cost of ballast.
However, as is now understood, HID lamp systems operating at relatively high frequencies suffer from the phenomena of acoustic resonance. Whether fluorescent-type systems suffer from the acoustic resonance phenomena is unknown, but it is believed that such fluorescent systems do not so suffer.
Acoustic resonance is a physical resonating of the mechanical elements of the lamp which disturbs the flow of energy through the gaseous medium in the lamp. There are many frequency regions or bands of acoustic resonance in HID lamps, attempted operation in some of which so disturb the flow of energy within a lamp that the lamp extinguishes. Operating in other frequency regions merely causes a flickering of the lamp, producing unpleasant visual effects.
Prior art teachings in connection with acoustic resonance have suggested to many that HID lamp systems may be designed for operation between frequency bands of acoustic resonance. This suggestion has proven unsatisfactory in that the frequency bands are somewhat unpredictable according to the various mixtures of gases within the lamp and according to specific geometric considerations of the lamp. Furthermore, it has been reported that studies for some HID lamps indicate that stable points beyond certain frequencies, such as 4700 Hz, are difficult to find, i.e., the frequency bands of acoustic resonance become closer and closer together and are less predictable. It is believed that such teachings have steered the prior art away from operating HID lamps, or even characterizing the precise effects of acoustic resonance, at ultra high frequencies on the order of greater than 60 kHz.
By virtue of the present invention, on the other hand, it has been discovered that an ultra-high frequencies, the frequency bands of acoustic resonance become less and less visually distractive. Accordingly, above a certain frequency, it appears that acoustic resonance does not exist since it is not visually disturbing, nor does operation at such a frequency extinguish the lamp or cause undesirable and unpleasant flickering of light.